1. Field of the Invention
The present invention relates to a scroll compressor, in particular, one suitable for operation in a vapour-compression refrigerating cycle which uses a refrigerant, such as CO.sub.2, in a supercritical area thereof.
2. Description of the Related Art
As for the vapour-compression refrigerating cycle, one of the recently proposed measures to avoid the use of Freon (fron, a refrigerant) in order to protect the environment is the use of a refrigerating cycle using CO.sub.2 as the working gas (i.e., the refrigerant gas). This cycle is called "CO.sub.2 cycle" below. An example thereof is disclosed in Japanese Examined Patent Application, Second Publication, No. Hei 7-18602. The operation of this CO.sub.2 cycle is similar to the operation of a conventional vapour-compression refrigerating cycle using Freon. That is, as shown by the cycle A .fwdarw.B.fwdarw.C.fwdarw.D.fwdarw.A in FIG. 3 (which shows a CO.sub.2 Mollier chart), CO.sub.2 in the gas phase is compressed using a compressor (A.fwdarw.B), and this hot and compressed CO.sub.2 in the gas phase is cooled using a gas cooler (B.fwdarw.C). This cooled gas is further decompressed using a decompressor (C.fwdarw.D), and CO.sub.2 in the gas-liquid phase is then vaporized (D.fwdarw.A), so that latent heat with respect to the evaporation is taken from an external fluid such as air, thereby cooling the external fluid.
The critical temperature of CO.sub.2 is approximately 31.degree. C., that is, lower than that of Freon, the conventional refrigerant. Therefore, when the temperature of the outside air is high in the summer season or the like, the temperature of CO.sub.2 at the gas cooler side is higher than the critical temperature of CO.sub.2. Therefore, in this case, CO.sub.2 is not condensed at the outlet side of the gas cooler (that is, line segment B-C in FIG. 3 does not intersect with the saturated liquid curve SL). In addition, the condition at the outlet side of the gas cooler (corresponding to point C in FIG. 3) depends on the discharge pressure of the compressor and the CO.sub.2 temperature at the outlet side of the gas cooler, and this CO.sub.2 temperature at the outlet side depends on the discharge ability of the gas cooler and the outside temperature (which cannot be controlled). Therefore, substantially, the CO.sub.2 temperature at the outlet side of the gas cooler cannot be controlled. Accordingly, the condition at the outlet side of the gas cooler (i.e., point C) can be controlled by controlling the discharge pressure of the compressor (i.e., the pressure at the outlet side of the gas cooler). That is, in order to keep sufficient cooling ability (i.e., enthalpy difference) when the temperature of the outside air is high in the summer season or the like, higher pressure at the outlet side of the gas cooler is necessary as shown in the cycle E.fwdarw.F.fwdarw.G.fwdarw.H.fwdarw.E in FIG. 3. In order to satisfy this condition, the operating pressure of the compressor must be higher in comparison with the conventional refrigerating cycle using Freon. In an example of an air conditioner used in a vehicle, the operating pressure of the compressor is 3 kg/cm.sup.2 in case of using R134 (i.e., conventional Freon), but 40 kg/cm.sup.2 in case of CO.sub.2. In addition, the operation stopping pressure of the compressor of this example is 15 kg/cm.sup.2 in case of using R134, but 100 kg/cm.sup.2 in case of CO.sub.2.
Here, a general scroll compressor comprises a casing; a fixed scroll and a revolving scroll in the housing, each scroll comprising an end plate and a spiral protrusion built on an inner surface of the end plate, said inner surface facing the other end plate so as to engage the protrusions of each scroll and form a spiral compression chamber. In this structure, the introduced working gas is compressed in the compression chamber and then discharged according to the revolving operation of the revolving scroll. The degradation of the operational ability of such a scroll compressor (using CO.sub.2 as the working gas and having high operating pressure) due to the leakage of the working gas may cause a problem. Therefore, in order to prevent such degradation, a floating structure is adopted, in which the fixed scroll can move only in its axial direction, and the back face of this fixed scroll is supported using a back pressure block.
In the above scroll compressor having the floating structure, it is necessary to form a discharge port (called "top clearance") of the compressed gas in the end plate of the fixed scroll and the back pressure block, and to attach a discharge valve at the outside of the back pressure block. Therefore, the clearance volume of the top clearance is large, and thus large recompressive force is necessary, thereby degrading the operational ability of the compressor.